Opening a wide window on the nano-world of surface catalysis

June 6, 2014
by Steven Powell

Two different etching methods produced two different kinds of nanoparticles: nanorice (top) and nanodumbbells. In both, the original flat surface of the nanocuboid was replaced by a curved surface with more exposed, catalytically active atoms. Credit: American Chemical Society

(Phys.org) —Surface catalysts are notoriously difficult to study mechanistically, but scientists at the University of South Carolina and Rice University have shown how to get real-time reaction information from Ag nanocatalysts that have long frustrated attempts to describe their kinetic behavior in detail.

The key to the team's success was bridging a size gap that had represented a wide chasm to researchers in the past. To be effective as nanocatalysts, noble metals such as Au, Pt, Pd and Ag typically must be nanoparticles smaller than 5 nm, says Hui Wang, an assistant professor of chemistry and biochemistry at South Carolina who led the team in collaboration with Peter Nordlander of Rice University.

Unfortunately, 5 nm is below the size threshold at which plasmon resonance can be effectively harnessed. Plasmon resonance is a phenomenon giving rise to a dramatic enhancement of impinging electromagnetic signals, which is the basis of analytical techniques such as surface enhanced Raman spectroscopy (SERS).

The ability to utilize the analytical power of plasmon resonance in a nanomaterial requires larger nanoparticles, "at least tens of nanometers in diameter," says Wang. The incompatibility of the two size regimes had long precluded the use of a range of spectral techniques based on plasmon resonance—SERS is just one—on noble metal nanocatalysts under 5 nm.

But as they just reported in Nano Letters, Wang and his team managed to combine the best of both size worlds.

Etching can be halted at different stages to produce a range of sizes and shapes of nanoparticles. Nanocuboids (top) and nanodumbbells at two different etching stages (middle and bottom) are shown here. Credit: American Chemical Society

Starting with cuboidal nanoparticles about 50 nm wide and 120 nm long, they chemically etched flat surfaces in a way that generated curved surfaces, creating nanoparticles that successfully catalyzed a model surface hydrogenation reaction. According to the team, the catalysis is the result of replacing low-energy atoms on the flat surface with exposed atoms after etching.

"If you have a flat surface, the coordination number of every single surface atom is either eight or nine," says Wang of their nanoparticles, which had a surface of pure Ag before etching. "But if you have some atomic steps on a surface, the coordination number will decrease. These exposed atoms are more active."

The stepped surface of the etched nanomaterial thus mimics the environment of a sub-5-nm nanoparticle: more exposed, active surface atoms can participate in catalysis.

And the catalysis is on a nanoparticle with plasmonic activity, which the researchers showed can be "tuned" by varying the shape and size of the nanoparticles. The team demonstrated the ability to convert cuboids (something like a short rod but with square rather than round sides) into what they termed "nanorice" and "nanodumbbells" through two different kinds of chemical etching. The two shapes had distinct plasmonic properties that could be varied by stopping the etching at different stages to create different sizes and shapes of nanoscale rice and dumbbells.

The spectral properties of the different nanoparticles are size- and shape-dependent and can be tuned by varying etching times. Credit: American Chemical Society

That plasmonic activity can be harnessed for SERS and other analytical techniques to study catalytic reactions in great detail as they occur.

"Raman spectroscopy is extremely powerful, with information about molecular fingerprints—you can see the structures, you can tell how the molecules are oriented on the surface," Wang says. "If you want to use GC, HPLC, or mass spec, you have to damage a sample, but here you can actually monitor the reaction in real time.

"And there is much more information with this approach. For example, we identified the intermediate along the reaction pathway. With those other approaches, it's really hard to do that."

Related Stories

(PhysOrg.com) -- Anyone who thinks amalgams are limited to tooth fillings is missing something: Amalgams, which are alloys of mercury and other metals, have been used for over 2500 years in the production of jewelry and for ...

(Phys.org)—From petroleum refining to food processing, the vast majority of commercial chemical applications involve catalysts to control the rate of chemical reactions. Anything that can increase the efficiency of catalysts ...

(Phys.org) —Plasmonic nanoparticles developed at Rice University are becoming known for their ability to turn light into heat, but how to use them to generate electricity is not nearly as well understood.

Silver (Ag) has a high catalytic activity towards many organic and inorganic transformations such as NOx reduction and catalytic oxidation of CO to CO2. In practical applications, catalysts like Ag are affixed to a substrate, ...

One of the challenges for the commercialization of polymer electrolyte fuel cells (PEFCs) is the development of new cathode catalysts for the oxygen reduction reaction (ORR) that exhibit superior activity and durability than ...

Recommended for you

Scientists have been making nanoparticles for more than two decades in two-dimensional sheets, three-dimensional crystals and random clusters. But they have never been able to get a sheet of nanoparticles to curve or fold ...

Serendipity has as much a place in science as in love. That's what Northeastern physicists Swastik Kar and Srinivas Sridhar found during their four-year project to modify graphene, a stronger-than-steel infinitesimally thin ...

Graphene has been called a wonder material, capable of performing great and unusual material acrobatics. Boron nitride nanotubes are no slackers in the materials realm either, and can be engineered for physical and biological ...

(Phys.org)—Currently, all light-emitting diodes (LEDs) emit light of only one color, which is predefined during fabrication. So far, tuning the color of light produced by a single LED has never been realized, despite numerous ...

When it comes to installing solar cells, labor cost and the cost of the land to house them constitute the bulk of the expense. The solar cells—made often of silicon or cadmium telluride—rarely cost more than 20 percent ...

A team of researchers from Berkeley Lab and Columbia University has passed a major milestone in molecular electronics with the creation of the world's highest-performance single-molecule diode. Working at Berkeley Lab's Molecular ...

0 comments

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.